The present invention relates to the control of the main stream audio rate in frames containing both VSB data and E-VSB data so as to limit audio related jitter in a receiver.
The ATSC digital television standard presently provides for the transmission of successive data fields each comprising 313 segments extending over a 24.2 ms time interval. FIG. 1 discloses an exemplary format for a data field according to this standard. The first segment of each field is a field sync segment. The field sync segment is composed of four two-level segment sync symbols and space for 828 other two-level symbols. A portion of this space is used for a field sync, and another portion of this field is reserved. Except for the reserved portion, and an alternating polarity 63 symbol PN sequence, the information in the frame sync segment does not change from field to field. Each of the remaining 312 segments (numbered 0 through 311) of each field comprises four two-level segment sync symbols and 828 n-level data symbols where n is currently eight, although n could be other integers such as two, four, sixteen, etc. Except for the segment sync portion, it is highly likely that the data in the remaining segments of the fields change from field to field.
As indicated by U.S. patent application Ser. No. 09/804,262 filed on Mar. 13, 2001, there is presently some interest in extending the ATSC digital television standard to allow a field to contain a mix of more robustly coded data (referred to herein as E-VSB data) and the data currently provided for in the standard (referred to herein as VSB data). Preferably, the data mix is employed on a segment-by-segment basis such that, ignoring the effects of interleaving in the encoder, some segments of a field are used to transmit VSB data exclusively and the remaining segments of the field are used to transmit E-VSB segments exclusively. However, it is possible that all data segments of a field could contain either E-VSB data segments exclusively or VSB data segments exclusively. Moreover, it is also possible that the E-VSB data contained in some segments of a field may be coded with one robust coding rate and that the E-VSB data in other segments of the field may be coded at other robust coding rates.
As disclosed in the above mentioned ""262 application, a map that indicates which segments contain the more robust (E-VSB) data and which segments contain standard VSB data is preferably provided by the transmitter to the receiver so that the receiver can properly decode and otherwise process the received VSB and E-VSB data. Assuming that a field contains E-VSB data at different coding rates, the map in that case must also designate the coding rates that apply to the differently coded E-VSB data segments.
The ""262 application describes one mapping system. Co-pending U.S. patent application Ser. No. 10/011,900 filed Dec. 3, 2001 as well as the ""333 application describe another mapping system that reliably identifies which segments contain first data (such as VSB data) and which segments contain second data (such as E-VSB data).
The ""953 application cited above discloses another map configuration in which maps are used to indicate allowed mixes of E-VSB segments in a field. The packing or distribution of these E-VSB segments in a field can produce jitter in the received signal. This jitter is caused by the non-uniform distribution of the E-VSB segments throughout the field. The ""191 application discloses how to pack or distribute E-VSB segments in a field so as to reduce jitter.
However, there is still a potential for packet jitter problems to occur in receivers with unrestricted use of E-VSB. The tradeoff of packets between the VSB streams and E-VSB streams can cause displacement of packets relative to a purely uniform spacing. This displacement tends to increase in the VSB stream as the percentage of E-VSB data segments increases. In other words, as the percentage of E-VSB data segments increases, the VSB data rate gets small, and when the VSB data rate gets small, the packets tend to group together. These packet bursts are a form of jitter which can cause problems with receiver decoder buffers. The present invention is directed to an implementation to reduce or eliminate this jitter.
In accordance with one aspect of the present invention, a method of transmitting a field comprises the following: generating a map for the field, wherein the map indicates the number of E-VSB segments and VSB segments in the field; inserting the map into the field; inserting E-VSB data into the E-VSB segments of the field; inserting VSB data into VSB segments of the field, wherein at least one of the VSB data and the E-VSB data includes audio data; limiting the audio data in the at least one of the VSB data and the E-VSB data to a predetermined data rate in response to the corresponding VSB data rate or E-VSB data rate being below a predetermined threshold; and, transmitting the field.
In accordance with another aspect of the present invention, a method of recovering data from a signal comprises the following: receiving the signal containing a field; demodulating the received signal to develop a demodulated signal containing a map indicating the number of E-VSB segments and VSB segments in the field, wherein the E-VSB segments contain E-VSB data, wherein the VSB data segments contain VSB data, wherein the VSB data includes audio data, and wherein the audio data is limited to a predetermined data rate in response to a VSB data rate below a predetermined threshold; de-formatting the field according to the map so as to separate the VSB segments and the E-VSB segments; and, processing the VSB data in the separated VSB segments in a VSB processor, the processing including buffering the VSB data in a buffer having a size which, in conjunction with the limit of the predetermined data rate, reduces audio jitter.
In accordance with still another aspect of the present invention, a receiver for recovering data from a signal comprises a tuner, a demodulator, a segment de-formatter, and a processor. The tuner is arranged to tune to the signal containing a field. The demodulator is arranged to demodulate the received signal to develop a demodulated signal containing a map indicating the number of E-VSB segments and VSB segments in the field. The E-VSB segments contain E-VSB data, the VSB data segments contain VSB data, the VSB data includes audio data, and the audio data is limited to a predetermined data rate in response to a VSB data rate below a predetermined threshold. The segment de-formatter is arranged to de-format the field according to the map so as to separate the VSB segments and the E-VSB segments. The processor is arranged to process the VSB data in the separated VSB segments in a VSB processor, and the processor includes a buffer having a size which, in conjunction with the limit of the predetermined data rate, reduces audio jitter.